In what follows, the terminology “GPS” (Global Positioning System) is often used. Depending on the context, such use should be understood to be either the version of GNSS (Global Navigation Satellite System) provided by the United States, i.e. NAVSTAR, commonly called GPS, or else any GNSS, such as e.g. GPS, Galileo, Glonass, SBAS (Space Based Augmentation System), LAAS (Local Area Augmentation System), or QZSS (Quasi-Zenith Satellite System). Also, the acronym “A-GPS” is often used here, and can mean either assisted GNSS, or specifically only assisted GPS.
A-GPS navigation uses an assistance server to cut down the time needed by a ranging receiver (i.e. a GPS receiver) to determine a location fix. A-GPS is useful in urban areas, where a user is often located in so-called urban canyons, or when a user is under heavy tree cover. A-GPS is becoming more common, and is usually associated with Location Based Services (LBSes) provided over cellular or other wireless networks.
A-GPS differs from ordinary (i.e. unassisted) GPS by including an assistance server in the overall system providing GPS signals. In (ordinary) GPS networks, a GPS receiver communicates directly with (receives signals directly from) GPS satellites, so there are only GPS satellites and GPS receivers. In A-GPS networks, an A-GPS receiver communicates with an assistance server, which communicates with (receives signals from) GPS satellites. (The A-GPS receiver might also receive signals directly from the GPS satellites.) The A-GPS receiver and the assistance server share computational tasks, reducing the computational burden on the GPS receiver, but the GPS receiver then has the additional task (compared with a pure GPS receiver) of performing cellular communication with the assistance server.
In a typical A-GPS network, an A-GPS receiver can ask an assistance server, located at what is often called a (serving) Mobile Location Centre (MLC), for the latest ephemeris for a satellite, or for corrections to an ephemeris to allow for a more accurate position fix determination by the A-GPS receiver. When asking for assistance regarding an ephemeris, the A-GPS receiver must unambiguously identify the version of the ephemeris. In the prior art the ephemeris is provided in a so-called L1 GPS signal, and the message providing the ephemeris includes a mechanism of unambiguously referring to the ephemeris. That mechanism is an IOD-E (Issue-of-Data-Ephemeris) information element (IE) within the message. (An IE can also be called a “field,” as in a “field of a record”). There are now so-called “modernized” GPS signals, referred to as L2C, L5 and L1C signals and described in IS-GPS-200D (Interface Specification-GPS-200D), IS-GPS705 and IS-GPS-800. These do not include an IOD-E.
Thus, what is needed is a mechanism useable in connection with the modernized GPS signals by which to unambiguously refer to an ephemeris conveyed by a modernized GPS signal.